Stereophonic sound-reproducing system

ABSTRACT

A stereophonic sound-reproducing system comprising two soundreproducing sets separated by a base distance and each having a high-frequency section and a mid frequency section. The mid frequency section is provided with an acoustic focuser, the acoustic axis of which is so oriented towards the base line as to make the perception of the spatial sound panorama practically independent of the listener&#39;&#39;s position on a line parallel to the base line.

United States Patent 1191 Borisenko [4 Sept. 18, 1973 s41 STEREOPHONIC SOUND-REPRODUCING 3,105,113 9/1963 018011 181/31 B SYSTEM 3,491,204 1/1970 Sherno 181/318 Inventor; Andrei Vladimirovich Borisenko,

prospekt Metallistov 80, Korpus 3 kv. 384, Leningrad, U.S.S.R.

Filed: Dec. 22, 1971- Appl. No.: 210,892

U.S. C1 .L 181/31 B, 179/1 GA lint. C1 G101 13/00, H041 1/28 Field of Search 181/31 B, 31 A;

179/1 GA, 1 GP References Cited UNITED STATES PATENTS Primary Examiner-Stephen J. Tomsky Attorney-Eric H. Waters et a1.

[57] ABSTRACT A stereophonic sound-reproducing system comprising two sound-reproducing sets separated by a base distance and each having ahigh-frequency section and a mid frequency section. The mid frequency section is provided with an acoustic focuser, the acoustic axis of which is so oriented towards the base line as to make the perception of the spatial sound panorama practically independent of thev listeners position on a line parallel to, the base line;

3 Claims, 3 Drawing Figures I Patented Sept. 18, 1973 2 Sheets-Sheet 2 FIG. 2

l STEREOPHONIC SOUND-REPRODUCING SYSTEM The present invention relates to radio engineering and, more specifically, to stereophonic soundreproducing systems.

axis of symmetry.

Even a slight deviation of the listener from the axis of symmetry distorts the sound panorama, as perceived by him, and most of the sound sources are picked up from the nearest sound reproducing set. If the listener is opposite one of the reproducing set, practically all the sound sources are picked up from that set. As a result, a sense of spatial sound panorama disappears and the perception is practically the same as monophonic. Thus, the zone, where the listener can.freely move without an appreciable loss of the sterophonic affect, is restricted to a narrow strip along the symmetry axis of the sound-reproducing sets.

Known in the art vis a stereophonic sound-' reproducing system, in which the stereophonic effect zone is expanded by using sophisticated large-sized reflecting devices, comprising more than two parabolic reflectors for differentsections of the frequency range.

Also known in the art is a stereophonic soundreproducing system comprising more than two soundreproducing sets. As the listener leaves the stereophonic effect zone of the main pair of soundreproducing sets, he enters the stereophonic effect zone of another pair of sets. A system comprising, for instance, four sound-reproducing sets takes much space in the room, for the sets are not set in a line while the increase in thefour set stereophonic system, as compared to the conventional stereophonic systems,

makes the system much costlier.

The main object of the invention is to produce a stereophonic sound-reproducing system which will considerably expand the stereophonic effect zone while eliminating the distortions of the spatial sound panorama.

This object is accomplished by providing the midfre quency section of the stereophonic sound-reproducing system with an acoustic focuser whose acoustic axis is so oriented with respect to the base line as to make the perception of the spatial sound panorama practically independent of the listener's position on a line, parallel to the base line.

One of the embodiments of the invention may be realized with an acoustic focuser designed as a laminar acoustic lens the plates of which are set at 40 relative to a 2.4-m long base line.

Another embodiment of the invention consists in having the acoustic axis of the high-frequency section oriented at approximately the same angle to the base line as that of the mid-frequency section.

A system, realized according to the present invention, with only two sound-reproducing sets makes it possible to expand the stereophonic effect zone practi- Also known in the art isa stereophonic soundreproducingsystem, wherein the stereophonic effect zone is increased by providing the walls of the audition room with special corrugated screens. As a result, the listeners pick up the sounds reflected from the corrugated screens. Thus the sounds, coming directly from the sets, are artificially attenuated, which brings about a drop in localization precision. g

Owing to their'complexity and large dimensions the above stereophonic sound-reproducing systems are usually ill-suited forindividual use.'

Known in the art is a stereophonic soundreproducing system, wherein an increase of the stereophonic effect zone is understood as the improvementof one of the components of the stereophony and which provides a constant overall loudness of both soundreproducing sets in a considerable part of the audition room. At-the same time no allowance is made for the effect of the time difference Ar, depending on the coordinates (x, y) of the listener's position. The effect of the time difference A7,, largely determines the distortionof the spatial sound panorama. I

An object of the present invention is to provide a system which avoids the aforesaid disadvantages.

cally to percent of the space in the audition room space. i

The invention will now be explained in terms of supportive theoretical data and described by way of exam-v plejwith reference to theaccompanying drawings, in which:

FIG. I isa top plan view showing leftand right-hand mid-frequency sections provided with a laminar acoustic lens:

FIG. 2 is a graphical illustration showing optimum directivity patterns of the mid-frequency (rightand lefthand) sections; and

FIG. 3 is a schematic diagram which illustrates the stereophonic effect zone where a is the stereophonic effect zone of existing systems and b is a stereophonic effect zone of the present system.

As is known, the various constituents of a stereophonic efiect, i.e. sound transparency, acoustic atmosphere and localization ensuring the subjective possibility of perception of spatially separated sound sources are closely interrelated. A violation of the rela tive arrangement of the various components of the stereophonic panorama with the listener in an asymmetric position (so-called spatial distortions) have an adverse effect on other aspects of stereophonic perception, too.

Moreover, an additional type of distortion appears violations of the musical balance which finds its expression in an exaggerated loudness of the sound sources located in the set, nearest to the listener. Therefore, the existing stereophonic sound -reproducing systems lose the advantages of stereophonyto a A7,, l, l lv;

the intensity difference AL, 201n 1 /1 the directively factor difference 20 In D, (til) 20 In D, (4') different spectra of the signals, received from the sound-reproducing sets at the listening point, determined by the frequency dependence of the directivity patterns of the sound reproducing set section, wherein: l, and l, are the distances from the listening point to the sound reproducing sets, V is the velocity of sound velocity, and D,(\li) and D (\l1) are the directivity patterns of the leftand righthand mid-frequency sections, respectively.

In the existing stereophonic sound-reproducing systems all the. above factors contribute to a still greater displacement of the apparent sources in the direction of the listeners displacement, the time difference Ar exercising the greatest effect.

It is known that the time (A7 and intensity (AL differences between two channel signals exercise an equivalent and mutually independent effect on binaural hearing. In other words, the overall displacement of the apparent source .r|/ A7111): a.-u) rv) A711: 0 ALI]! The intensity difference, equivalent in effect to the time difference, equals where k is the equivalence coefficient in db/msec dependent on the co-ordinates of the listening point.

Then, the overall intensity difference with due regard to the equivalent will equal AL, total AL, k A1,,

' Thus, an increase of the delayed signal level may offset the effect of the time difference Ar on the aural sense of direction. This may be accomplished with the aid of certain directivity patterns of the soundreproducing set sections. 1 I

The optimum directivity patterns of the sections may be derived from the condition that the position of one of the apparent sound sources, e.g., the central one, remains unchanged as the listener moves parallel to the base line.

This condition may be roughly expressed by the following equation AL k- A1,, 201n D, (110- In D (II!) 0 The optimum directivity patterns, rated for different bases and presented in FIG. 2, illustrate the parts of the directivitypatterns which correspond to the direction towards the listener's positions. For other directions (for 60) the shape of the directivity patterns may be arbitrary from the standpoint of the stabilization of the spatial sound panorama. It is, however, desirable that the values of the directivity factorsfor these directions be as small as possible.

The directivity of an individual mid-frequency section is much lower than the optimal value. Therefore, an acoustic focuser is required in the mid-frequency range. The required directivity of the mid-frequency section in a horizontal plane is ensured by using, for example, a laminar acoustic lens the design of which depends on the length of the base line selected.

The presentstereophonic sound-reproducing systeming filters. The overall width of the frequency range, reproduced by the present system, is determined by the selection of highand low-frequency speakers.

The mid-frequency sections 1 (FIG. 1) comprising two speakers 2 each, are supplied with a laminar acoustic lens 3. With base B 2.4 m long, the acoustic lens 3 has between six and eight parallel plates 4, set at an angle of 40 relative to the base line and diminishing in length in a direction outwardly along the base line. The laminar acoustic lenses 3 impart to the mid-frequency sections 1 certain directivity patterns D,(and D441), shaped on the basis of studying the characteristics of mans aural analyzer. Optimum directivitypatterns 5, 6 and 7 for the left-hand mid-frequency section and 8, 9, 10 for the right-hand mid-frequency section are estimated depending on different base lengths and illustrated in FIG. 2.

The difference in directivity factors enables the listener at an arbitrary point A in the audition room to pick up the signals from the distant mid-frequency section at a higher level than from the nearer section. The magnitude and sign of this difference are calculated to compensate for the effect on the hearing organ of the time difference and the intensity difference due to the difference in the distances 1, and 1 from the point A to the mid-frequency sections. As a result, the position of each of the apparent sound sources in the stereophonic panorama is practically independent of the listeners position. 1

It should be also noted that the required directivity at high frequencies between3 and 5 kHz may be provided by the orientation of the acoustic axis of the highfrequency section, shown in dotted outline by numeral 11 in FIG. .1 at roughly the same angle as that of the mid-frequency section. At the same time it is desirable to reduce excessive directivity in the higher-frequency range (over 5 kHz).

FIG. 3 illustrates the boundaries of the stereophonic effect zone within which the spatial distortions do not exceed 10 percent, where a-is the stereophonic effect zone of the existing systems and b-is the stereophonic effect zone of the present system. This means that the central apparent source of the perceived spatial sound panorama is displaced by not more than 10.18. As may be seen from FIG. 3, the present system makes it possible to expand the stereophonic effect zone up to percent of the audition room space.

The present system was tested and produced favorable results. A series of comparative auditions of the herein proposed and existing stereophonic systems was made, and as a result, the following was established:

1. The present system produced a stable stereophonic panorama for listeners both on the axis of symmetry of the sound-reproducing sets and away from it.

2. The sound quality of this system is preferred by the vast majority of listeners who are off the symmetry axis of the sound reproducing sets and is estimated as equivalent to the existing stereophonic sound-reproducing systems when positioned on the axis of symmetry. The advantages of the present system are displayed particularly in reproducing stereophonic programs with a complex spatial panorama (e.g., symphonic music) as well as at listening points opposite one of the sound reproducing sets (leftor right-hand).

3. The loudness ratio of the various elements of the stereophonic panorama (the musical balance) remains practically unchanged within a considerable part of the room.

4. For this system there is no appreciable difference between the perception of stereophonic sound on the symmetry axis of the sound reproducing sets and aside from it.

What is claimed is:

l. A stereophonic-sound reproducing system comprising: two sound-reproducing units symmetrically positioned on a base line and separated by a base distance, each unit having a mid-frequency section and a high-frequency section, and acoustic focussing means positioned in front of said mid-frequency section, each mid-frequency section having an acoustic axis inclined with respect to the base line, the acoustic axes of both mid-frequency sections facing inwardly in converging fashion and interesecting on an axis of symmetry of the units, each acoustic focussing means comprising a plurality of spaced vertical plates extending parallel to the acoustic axis of the respective mid-frequency section, said plates having a length in a horizontal plane which decreases with increased distance of the plates from said axis of symmetry, whereby to make the perception of the spatial sound panorama produced by said units practically independent of the listeners position on a line parallel to the base line.

2. A stereophonic sound-reproducing system as claimed in claim 1, wherein said plates are disposed at an angle of 40 relative to said base line, the latter being 2.4 m long.

3. A stereophonic sound-reproducing system as claimed in claim 1, wherein each said high-frequency section is oriented relative to the base line so that the acoustic axis of said high-frequency section is at approximately the same angle to the base line as that of the associated mid-frequency section. 

1. A stereophonic-sound reproducing system comprising: two sound-reproducing units symmetrically positioned on a base line and separated by a base distance, each unit having a midfrequency section and a high-frequency section, and acoustic focussing means positioned in front of said mid-frequency section, each mid-frequency section having an acoustic axis inclined with respect to the base line, the acoustic axes of both mid-frequency sections facing inwardly in converging fashion and intersecting on an axis of symmetry of the units, each acoustic focussing means comprising a plurality of spaced vertical plates extending parallel to the acoustic axis of the respective midfrequency section, said plates having a length in a horizontal plane which decreases with increased distance of the plates from said axis of symmetry, whereby to make the perception of the spatial sound panorama produced by said units practically independent of the listener''s position on a line parallel to the base line.
 2. A stereophonic sound-reproducing system as claimed in claim 1, wherein said plates are disposed at an angle of 40* relatiVe to said base line, the latter being 2.4 m long.
 3. A stereophonic sound-reproducing system as claimed in claim 1, wherein each said high-frequency section is oriented relative to the base line so that the acoustic axis of said high-frequency section is at approximately the same angle to the base line as that of the associated mid-frequency section. 